Snippets from other packages/ License

I want to use a function from another package (which is GPL>=3),
about 20 lines of code, in my own package. I somewhat hesitate to
depend on the entire package just for this single function, but of
course I want to credit the original authors.

What would be the best way to do that? Where should I put that
credit? Or should I proceed completely different?

You could use 'Imports' if you will always use that code for your package.

You could use 'Suggests' with appropriate tests to see if it is
available if you only need it occasionally, or for only one function.

If you wish to fork the original code and include the code directly in
your package, then your package will also need to be GPL>=3, you will 
need to list the authors of that code as Contributors in your 
DESCRIPTION file, and I would strongly recommend that you place this 
code in a separate .R file in your package, with a separate Copyright 
and License comment block in that separate file stating the origins and 
license for this code snippet.

That should satisfy both the spirit and the letter of the GPL.

Cheers,

Brian

Brian G. Peterson
http://braverock.com/brian/
Ph: 773-459-4973
IM: bgpbraverock